The present invention relates generally to motor operated valves and, more particularly, to a control for switching off the motor of a motor operated valve when a predetermined compressive thrust is achieved within the valve stem.
Most motor operators that are employed to open and close motor operated valves, such as gate or globe valves, are controlled on the basis of the torque developed as a result of opening or closing of the valve. Motor operated valves of this type are commonly used in the utility or power generating industry and are sometimes employed as safety valves for the protection of power equipment, as well as for the protection of utility personnel and/or the general public, from the release of hazardous materials, either directly or indirectly. In general, a predetermined amount of thrust, not torque, is required to ensure that a valve of this type will properly close against severe differential pressure that may occur under an accidental or emergency condition that the valve is intended to mitigate. Thus, to the extent that the torque setpoint employed to control operation of the valve may vary from the desired thrust setpoint, a motor operated valve being controlled exclusively on the basis of torque may not achieve the level of thrust necessary for the valve to properly and/or fully close under either normal or emergency operation and, thereby, may not completely stop the flow of material through the valve. For reasons which will hereinafter be discussed in greater detail, the amount of thrust applied for a given amount of torque may vary significantly, depending upon the amount and condition of the lubricant within the valve operator or actuator, particularly, the lubricant between the valve stem and the stem nut that is driving the valve stem. In addition, with motor operated valves of the type presently employed, the amount of torque developed can vary significantly, depending upon the rate of loading. Since the rate of loading can vary substantially between a test condition where the setpoint is usually established and where no differential pressure is applied and actual operating conditions in which a substantial differential pressure may be present even with no changes in lubrication, the level of thrust can vary under operating conditions.
Clearly, a need exists for an improved control system for controlling a motor operated valve of this type, particularly for insuring that the valve is tightly closed before turning off the operator. Ideally, such a control means should be based upon thrust rather than torque and should not be affected by changes in the rate of loading. In addition, such a control system should be broadly applicable to all commonly used motor operators and motor operated valves and should be easily retrofitable to motor operated valves of this type which are already in place, particularly valves of this type within the nuclear utility industry which is most adversely affected by the above-discussed problems inherent in motor operated valves controlled on the basis of torque.
A major manufacturer of motor operators used in such valves, Limitorque Corporation, recently presented a paper to concerned nuclear industry specialists. The paper, entitled "Field Modification of Limitorque Actuators for Control of Valve Seating Thrust," the substance of which is incorporated herein by reference, suggested a change over to thrust control of such motor operators and outlined a proposed modification to achieve the change over. While the proposed modification would facilitate control of such motor operated valves based upon thrust rather than torque, the methodology and equipment proposed in this paper clearly could not be employed with some commonly used motor operated valves. In addition, because of size and weight constraints, the equipment proposed in this paper could not be readily retrofitable on most existing valves and valve operators.
The present invention, as described in greater detail below, overcomes these drawbacks of the proposal set forth in the paper by providing a thrust actuated control for a motor operated valve, which is not affected by changes in the rate of valve loading. The present invention also has broad applicability with respect to all commonly used motor operators and is easily retrofitable on existing valve operators without major system modification.